How do I use Ohm’s law?
Fill in any two of voltage, current, resistance and power. The calculator finds the other two with Ohm's law.
- Current
- 3 A
At 12 V across 4 Ω, the current is 3 A and the power is 36 W.
- Power
- 36 W
Current: 3 A. At 12 V across 4 Ω, the current is 3 A and the power is 36 W.
How to calculate
Computes voltage, current, resistance and power from any two of them, with Ohm's law (V = I × R) and electric power (P = V × I).
Example with the default inputs (Voltage 12 V, Resistance 4 Ω): At 12 V across 4 Ω, the current is 3 A and the power is 36 W.
Formula: V = I × R (Ohm’s law) and P = V × I, so also P = I² × R = V² ÷ R.
- The resistance is ohmic: it stays the same whatever the voltage or current.
- Values are for direct current, or root-mean-square (RMS) values in a purely resistive AC circuit.
- All values are positive.
Worked examples
Each example is checked against the calculator on every build.
- Voltage 12 V, Resistance 4 Ω gives Current 3 A, Power 36 W.Source: hand calculation in content.mdx: I = 12 V ÷ 4 Ω = 3 A; P = 12 V × 3 A = 36 W
- Power 60 W, Voltage 120 V gives Current 0.5 A, Resistance 240 Ω.Source: hand calculation in content.mdx: I = 60 W ÷ 120 V = 0.5 A; R = 120 V ÷ 0.5 A = 240 Ω
- Power 0.25 W, Resistance 1,000 Ω gives Voltage 15.81 V, Current 0.01581 A.Source: hand calculation in content.mdx: V = √(0.25 W × 1,000 Ω) = √250 = 15.81 V; I = 15.81 V ÷ 1,000 Ω = 15.81 mA
- Current 0.02 A, Resistance 220 Ω gives Voltage 4.4 V, Power 0.088 W.Source: hand calculation in content.mdx: V = 0.02 A × 220 Ω = 4.4 V; P = 4.4 V × 0.02 A = 0.088 W
How it works
Two equations link the four quantities:
- Ohm's law: V = I × R
- Electric power: P = V × I
Here V is the voltage in volts (V), I is the current in amperes (A), R is the resistance in ohms (Ω), and P is the power in watts (W). By the SI definitions, 1 Ω = 1 V/A and 1 W = 1 V × 1 A.
Put one into the other and you get two more forms of the power law:
- P = I² × R
- P = V² ÷ R
Give any two of the four values and the calculator finds the other two:
| You know | Formulas used |
|---|---|
| V and I | R = V ÷ I, P = V × I |
| V and R | I = V ÷ R, P = V² ÷ R |
| I and R | V = I × R, P = I² × R |
| P and V | I = P ÷ V, R = V² ÷ P |
| P and I | V = P ÷ I, R = P ÷ I² |
| P and R | V = √(P × R), I = √(P ÷ R) |
Units such as mV, kV, mA, µA, kΩ, MΩ, mW and kW are converted to volts, amperes, ohms and watts first, with the SI prefixes (m = 10⁻³, µ = 10⁻⁶, k = 10³, M = 10⁶).
Assumptions
- The resistance is constant (an ohmic resistor).
- Values are for direct current, or RMS values in a purely resistive AC circuit.
- Every value is greater than zero.
Worked examples by hand
12 V across 4 Ω. I = 12 ÷ 4 = 3 A. P = 12 × 3 = 36 W.
A 60 W bulb on 120 V. I = 60 ÷ 120 = 0.5 A. R = 120 ÷ 0.5 = 240 Ω.
A 1 kΩ resistor at its 1/4 W rating. V = √(0.25 × 1,000) = √250 = 15.81 V. I = 15.81 ÷ 1,000 = 0.01581 A = 15.81 mA. This is the highest voltage the resistor takes without going over its power rating.
20 mA through 220 Ω. V = 0.02 × 220 = 4.4 V. P = 4.4 × 0.02 = 0.088 W (88 mW).
Other questions people ask
What is Ohm's law?
Ohm's law says that the current through a resistor is proportional to the voltage across it: V = I × R. V is the voltage in volts, I is the current in amperes, and R is the resistance in ohms. One ohm is one volt per ampere.
How do I find the current from the voltage and the resistance?
Divide the voltage by the resistance: I = V ÷ R. For example, 12 V across 4 Ω gives 12 ÷ 4 = 3 A.
How do I work out power in watts?
Multiply the voltage by the current: P = V × I. With Ohm's law you can also use P = I² × R or P = V² ÷ R. A 60 W bulb on 120 V draws 60 ÷ 120 = 0.5 A.
Why does the calculator need two values?
There are two independent equations (V = I × R and P = V × I) and four quantities. Two known values fix the other two. If you enter three or four values that do not fit the equations, the calculator says which ones disagree.
Does Ohm's law work for AC circuits?
For a purely resistive load, yes, if you use root-mean-square (RMS) voltage and current, which is what a mains voltage such as 120 V or 230 V states. Circuits with capacitors, coils or motors have reactance, and then you need impedance instead of resistance.
Which parts do not follow Ohm's law?
Parts whose resistance changes with the current, such as diodes, LEDs and filament bulbs as they heat up. For those, V = I × R holds only at one working point.